Greenbriar suppression with goat mob browsing

L L Boggs, J P Muir* and J W Dunn

Abstract

Little information is available on the suppression of greenbriar (Smilax
spp.), an invasive native vine of North America, by goats. Goats will
readily browse greenbriar but little is known about how permanent this
vegetation removal is and if it positively affects the subsequent
development of the herbaceous canopy. When land managers are unwilling
to spray herbicides or unable to afford chemical or mechanical means of
controlling invasive plants, they may be willing to hire goats to achieve
the same goal. Wooded paddocks in north-central Texas were split into
mechanically cut (to allow goat access to all greenbriar leaves) and uncut
(goats given access only up to 2 m).

Days to 95% leaf removal of 12 kids/ha
required an average 48 days while paddocks with 24 and 36 kids/ha averaged
30 days (P<0.05). Long term suppression of greenbriar did not
vary by stocking rate or mechanical cut (P>0.05). However,
herbaceous grass percentage of herbage mass did increase (P< 0.05) in
all browsed paddocks compared to unbrowsed areas. Our results indicate that
complete removal of greenbriar leaves can be achieved within a short time
but destruction of plants may require long term browsing or use of
herbicides on regrowth following browsing. Short duration mob browsing
may provide flock owners a source of income in exchange for ridding land of
unwanted greenbriar herbage, but suppression will only be temporary since
plants are not destroyed during such short periods of herbage removal.

Keywords: biological control, mechanical control, Smilax

Introduction

Kiely et al (2004) estimated that in 2000, U.S. agriculture producers applied $5
million in herbicides to kill unwanted plants (Table 1). Not only is this a
costly venture, it can also lead to weed resistance after repeated
applications and exposure of non-target populations to harmful chemicals.
Alternative biological methods for weed control include the use of ungulates
in non-crop situations such as forested areas.

Using goats
for brush control is not a new strategy. As early as the 1950’s and 1960’s
goats were being recognized as effective and economical in control of brush and
small hardwoods in pasture areas (Magee 1957; Wilson 1969). In
addition, Luginbuhl et al (1999) indicated goats are highly effective and
economical biological control agents. Studies conducted over four growing
seasons showed controlled defoliation by goats increased desirable vegetative
cover in experimental plots from 65 to 85% while at the same time multi-flora
rose bushes (Rosa multiflora Thumb.) were practically eliminated.

Browsing
habits of goats have produced similar results with other noxious weedy plants.
Toranno et al (1999) indicated 75-80% thistle control was achieved using goats
at lower than maximum desired stocking rates. The recommended stocking
rate of goats is 1 goat/ha for every 3% increase in thistle population to
achieve desired maximum thistle control. Only 17 goats were used in the 7
ha study done by Toranno et al (1999) when the recommended maximum stocking rate
was 42 goats.

In Brazil,
goats select browse and forbs over grass during both wet and dry seasons
(Pfister and Malechek 1986). Because of this preference, they have been
successfully used to turn Brazilian caatinga, where woody vegetation dominates,
into grasslands where cattle thrived Pfister and Malechek 1986). In the
U.S.A., Luginbuhl et al (1996) reported that goat browse studies conducted in
hardwood forests of Arkansas showed 65% of goat diet from July to December
consisted of vines, and the vines remained an important part of the goat diet
year round.

Several
studies conducted in Texas have indicated that stocking rate appears to have
both positive and negative effects on weed and brush control. Mellado et
al (2003) reported that heavily stocking pastures resulted in negative effects
on both the land and the goats browsing it. Goats were forced, due to
overgrazing, to alter their diet selection and consume more resinous, toxic and
coarse species of plants resulting in negative daily weight gains and lower
fertility. In addition, shrub and grass cover in overgrazed pastures was
reduced. Owens et al (1991) determined that stocking rates of 0.8 goats/ha
appeared optimum for brush control experiments in Texas.

Very little
information is available on the control of greenbriar, which is considered to be
one the most troublesome weeds throughout much of northeastern and southeastern
North America (USDA, 2011; Figure 1). Goats will readily browse greenbriar
(Packard et al 2007) but suppression and regeneration dynamics are poorly
understood. This study was initiated to evaluate the control of greenbriar using
various goat stocking rates.

Figure 1: Greenbriar (Smilax sp.) is one of the
most troublesome plants goats can help control in North America.

Figure 2: Artificially low browse lines were created by
cutting greenbriar at ground level in one half of each paddock. Goats were
allowed to establish their own browse lines in the other half of the paddock.
Browse lines were only created in May and June of the first year of grazing.

Each paddock
was divided in two. In half of each paddock, brush was left for goats to
establish their own browse line; goats in these areas could only access
greenbriar leaves up to 2 m, the approximate browse line. In the other half,
brush (but not trees) was mechanically cut to 0.5 m using chainsaws, hedge
trimmers and pruning instruments to guarantee goats access to all greenbriar
leaves (Figure 2). Initial
greenbriar populations were sampled in May and June 2005 to establish a baseline
vegetation survey in the mechanically cut and uncut halves of each paddock.
Individual plant counts and foliage dry matter (DM) were quantified on both cut
and uncut sides of the paddocks in 10 random locations on each side.
Stakes were set in the center of each sample location and a 0.5 m string was
attached. Plant counts were taken from the circle around the stake within
this radius up to a height of 2 m to include any browse goats might consume.
In the second and third years of the study, 10 locations within each half
paddock were randomly sampled in the same manner. Plants collected were
separated by species, leaves removed from stems and dried for 48 hours at 55o
C. Dry matter weights were taken to determine herbage mass available per
individual plant of each species in each half-paddock. Only leaf samples
were used to determine dry weight and nutritional value as the goats generally
only consume the leaves.

Initial plant
composition was widely variable among paddocks. While all eight paddocks
contained the same basic group of plants (greenbriar, grass, hardwood trees and
broadleaf species, mainly nightshade, thistle and noseburn), they were found in
differing numbers in the individual paddocks. For example, in initial
plant sampling, Paddock 9 had nine distinct plant species identified in the
browse zone. Two of these samples showed different masses, 225 kg/ha of
greenbriar but only 56 kg/ha of grass. In contrast,
Paddock 4 contained only four distinct species of plants in which greenbriar
comprised 103 kg/ha and grasses 170 kg/ha.

Boer x
Spanish kid goats weighing approximately 20-25 kg were purchased from local
markets and private producers. All goats were weighed, wormed and then
separated into groups to be placed into the paddocks. Goat stocking rates
of 0 (control), 12 (low stocking rate), 24 (medium stocking rate) and 36
goats/ha (high stocking rate) were established in two paddocks each for a total
of eight paddocks (Figure 3). Once goats were turned onto paddocks, they
only received multi-mineral block supplement and water. All goats were
removed from their respective paddocks when the browse was visually determined
to be 95-100% cleared (Figure 4).

Long-term
rainfall average from May to October is 452 mm at the trial site. However,
in 2005 it was only 61% (275 mm), in 2006 it rose to 68% (308 mm) and in 2007 it
was 137% (623) of this long-term average during the same 6-mo growing season.

Analysis of
variance (ANOVA) was run on greenbriar percentage of herbage and grass
percentage of total herbage up to a 2-m height for 2005, 2006 and 2007.
Stocking rate and mechanical cutting were considered independent variables and
the interaction of these two factors were tested and simple effects submitted to
ANOVA only when this interaction was found to be not significant at P<0.05.
Where appropriate, individual mean separation using least square differences (P≤0.05)
were calculated. Years were not considered a factor in the model since effects
of browse, which were imposed on the same paddock every year, would be
cumulative.

Figure 3: Boer by Spanish kids were stocked at the
rates of 0, 12, 24, and 36 goats/ha in 0.4 ha paddocks. Paddocks 5 and 10
were control paddocks with 0 goats per acre. Paddock 1 was the home
paddock and paddock 6 was not used.

Figure 4: Goats were removed from their respective
paddock when plant material was 95% to 100% cleared.

Results

Goat stocking
rate affected the number of days it took goats to clear the paddocks (Figure 5).
While goats stocked at 24 and 36 goats/ha had little difference in days to
clear, they took less time (P=0.005) than those stocked at 12 goat/ha.
Goats stocked at 12 goats/ha took an average of 48 days to clear their 0.4 ha
paddock, while those stocked at 24 and 36 goats/ha required only 30 days, on
average, to clear the same area (Figures 6 and 7). Differences (P=0.009)
were also noted between the number of goats per hectare and the year of the
study as well. In 2006, when 308 mm rainfall fell from May to October, the
trial area experienced a severe drought resulting in a shorter number of days,
regardless of stocking rates, to clear brush compared to 2005 (when plants had
accumulated reserves from previous years) or 2007 when 623 mm rainfall fell from
May to October compared to the 452 mm long term rainfall.

Figure 5: As number of goats per paddock increased,
the number of days to 95% greenbriar leaf removal declined (P=0.005; LSD
7.1).

Figure 6: Twelve goats/ha (left) took 46 days to
clear a 0.4 ha paddock. Twenty-four goats per hectare (right) took just 30
days to clear the same area.

Figure 7: Thirty-six goats/ha effectively cleared
the paddock on the left in an average of 28 days. The paddock on the right
is a control paddock which contained no goats.

Greenbriar
as a percent of total herbage, did not show a decline over the 3-yr study
period (P>0.05) (Figure 8). Control paddocks that were not browsed
did show an increase in greenbriar percentage (P<0.05), however.
Percent greenbriar tended to increase in 2006, likely a reflection of decreased
contribution of the herbaceous component as a result of low rainfall (308 mm)
from May to October compared to 623 mm during the same period in 2007.

Figure 8: Percent of herbage mass up to 2 m height
that greenbriar (Smilax spp.) contributed per paddock did not show a
decline over the 3-yr study period (P>0.05). Greenbriar percentage
in paddocks that were not browsed increased (P<0.05; LSD 16.5).

Removing
browse should allow more sunlight and growing space for herbaceous species.
However, no increase was observed over the study period (P>0.05; Figure
9). Control paddocks did show a decrease (P<0.05) in percent grass over
the three year study period relative to paddocks browsed by goats. There
was a tendency for greenbriar percentage of herbage mass to increase relative to
herbaceous species in 2006, a dry year, likely because deep roots and nutrient
storage in the roots and vines gave the perennial greenbriar an advantage
vis-à-vis the more shallow-rooted herbaceous grasses. When rainfall increased in
2007, this trend was reversed.

The cutting
treatment produced no effect (P>0.05) in the decrease of leaf regrowth
potential during the three years of the study. In fact, by the end of the
study many plants that had been cut to allow full access of leaf growth to
browsing goats had grown vines above 2 m and beyond the reach of the animals in
subsequent years. This indicates that short-duration exposure to browsing, even
when all leaves are within browsing range, is insufficient to permanently
suppress greenbriar.

Figure 9: Percent grass in the paddocks did not show
a decrease (P>0.05) over the 3-yr study period when browsed by goats at
three stocking rates. Control paddocks which were not browsed showed a
decrease (P<0.05; LSD 15.2) in percent grass

Conclusions

Goats
provide a means of vegetation control in areas where conventional methods are
not practical or are too costly. Effective removal of problem species can
be accomplished in as little as 25-30 days with 24 goats/ha stocking rates.
Goats will readily browse the brush, hardwood and broadleaf plants, allowing
grasses, which tend not to be a plant of choice when mature, to thrive and set
seed. Although goats provide a means of greenbriar leaf removal, it is
important to note that they do not eradicate the plants even after three years
of intensive, short-duration browsing during the peak of greenbriar growing
season. The regrowth will come back, sometimes in as little as 5-7 days
after goats are removed from a grazing area. This indicates that
biological control may require repeated rotational or even continuous browsing
pressure. Another option that should be tested is the improved efficacy of
herbicide application on greenbriar regrowth as opposed to application of
herbicide on more mature leaves on plants that are not browsed.

Cutting
greenbriar at ground level allowed goats access to all leaves produced by
plants. However, this did not increase destruction of greenbriar plants as
measured by their ability to produce leaf material. When compared to greenbriars
whose vines escaped browsing above the 2-m browse line, those plants that were
artificially lowered to within goat browsing range did not experience a decrease
in leaf production after three years of browsing pressure.

Luginbuhl J-M, Harvey T E, Green Jr. J T, Poore M H and
Mueller J P 1999 Use of goats as biological
control agents for the renovation of pastures in the Appalachian region of the
United States. Agroforestry Systems 44:241-252.